Recently, a water-absorbent polymer having an excellent absorption property has been developed and applied for various purposes. For example, the water-absorbent polymer has been used in a sanitary material such as a disposable diaper and a sanitary napkin; a pet sheet; a water stop material; or the like.
As a raw material of such a water-absorbent polymer, a variety of monomers and hydrophilic polymers are proposed. Among those, a polyacrylic (polyacrylate) water-absorbent polymer obtained by using acrylic acid and/or its salt as a monomer has been mostly produced and used in view of cost performance. For example, in a case of the sanitary material such as the disposable diaper and the sanitary napkin, a powdered water-absorbent polymer is mixed with white-colored pulp for use. In this case, in view of consumer acceptability, the water-absorbent polymer is required to be white for making a clean impression.
Generally, the water-absorbent polymer is in a form of white powder when it is shipped. However, the water-absorbent polymer is known to be colored (from white to yellow-brown) before it reaches consumers in a form of the disposable diaper or the like, for example if it is exposed under a condition of high humidity and high temperature for long periods of time. Therefore, it is desired to develop a water-absorbent polymer that excels in long-term color stability even over long-term exposure under the condition of high humidity and high temperature.
In order to prevent such a coloration, there have been known such methods as: a method of polymerizing an acrylic acid monomer and/or its salt with hydroxyperoxide and a reductant and then treating the resultant polymer with a silane coupling agent (Patent Literature 1); a method of adding an organophosphorus acid compound or its salt to a water-absorbent polymer after polymerization (Patent Literature 2); a method of controlling a total amount of hydroquinone and benzoquinone contained in acrylic acid to 0.2 ppm or less (Patent Literature 3); a method of adding an inorganic reductant to a water-absorbent polymer (Patent Literatures 4 and 5); a method of adding organic carboxylic acid or its salt to a water-absorbent polymer, and further adding an inorganic reducing agent or the like to the water-absorbent polymer (Patent Literatures 6 to 8); a production method in which polymerization is carried out with use of tocopherol as a polymerization inhibitor contained in acrylic acid (Patent Literature 9); and a production method in which a metal chelator is added in producing a water-absorbent polymer (Patent Literatures 10 and 11).
Among the metal chelators, a phosphorous chelator is used not only for preventing the coloration, but also is proposed to be used in other production methods, such as a production method of a water-absorbent polymer in which the phosphorous chelator is used as a polymerization stabilizer of a reversed phase suspension polymerization (Patent Literature 12) and a production method in which the phosphorous chelator is used as a gel stabilizer (Patent Literature 13).
However, none of the above-described methods could make a sufficient improvement in the coloration, and they rather have had such problems as a property deterioration, a cost increase, and depending on a compound to be used, a safety problem. Thus, (i) an absorbing property and a urine tolerance and (ii) the long-term color stability have conventionally been in such a trade-off relationship.
Further, in a case where the water-absorbent polymer is dried at a high temperature or surface-crosslinked at a high temperature (for example, 150° C. or more), the water-absorbent polymer is often colored or thermally deteriorated (for example, extractable content increases). Lowering a drying temperature or a temperature at which the surface crosslinkage is performed so as to avoid the above problems will be associated with a decrease in productivity.